Global Navigation Satellite Systems (GNSS) such as Global Positioning System (GPS) are widely used in order to provide accurate location information to support navigation. However, GNSS is not always available or it can be severely degraded, for example inside buildings, underground or when it is actively jammed. In these circumstances other technologies are required in order to provide an accurate real-time estimate of an individual's location to both the user and to an external observer. Example applications include firemen working in reduced visibility inside buildings which are on fire and special forces soldiers conducting a night-time raid in a large and complex building. In both cases the individuals need accurate information on their position or location as do any commanders outside the building overseeing activities.
One approach to estimating location in the absence of GNSS is to utilise dead-reckoning. A dead-reckoning system attempts to determine an object's location through estimating the displacement from a known starting point. This is usually achieved through the integration of sensor data over time.
An example dead-reckoning system is described by Hutchins (U.S. Pat. Nos. 5,724,265; 5,899,963; 6,122,960; 6,305,221) and involves the emplacement of inertial sensors (accelerometers and gyroscopes) within an individual's shoe. By integrating the accelerometer and gyroscope data over time, the displacement of the shoe from a known start point can be estimated. The location is displayed on a wristwatch wirelessly connected to the shoe-mounted navigation system.
The positional accuracy of such dead-reckoning systems, however, tends to degrade with both time and distance traveled due to the accumulation of errors. This limits their applicability, especially when high positional accuracy is required over a long period of time, or distance traveled, or both.